This invention relates to a solar heat collector.
There are generally two kinds of solar heat collectors in the art: one wherein a proper vehicle (for example, water) is supplied and circulated within a rectangular box and the other wherein a narrow copper tube is inserted into a cylindrical vacuum glass tube for passage of a proper vehicle. When it is desired to achieve air-conditioning through the utilization of solar heat energy, the latter vacuum tube type is the only effective way in elevating the temperature of the vehicle to a desired value. Accordingly, the vacuum tube type of solar heat collectors seems to be more promising but has still a variety of problems especially in conjunction with manufacturing techniques and operating life.
More particularly, reference is now made to FIGS. 1 and 2 to give a better understanding of these aspects. In FIGS. 1 and 2, there are illustrated an outer glass tube 1 which serves as a transparent cover and a casing and a cylindrical fin structure 2 which comprises a selective absorbent overlaying at least on its outer surface and is made of typically aluminum by well-known extrusion molding. A heat-collecting pipe 3 of typically copper in heat conducting contact with the cylindrical fin structure 2 provides a passage for the vehicle which is to be heated by depriving the cylindrical fin structure 2 of heat absorbed therein. A sealant 4 is provided at one end of the outer glass tube 1. In order to secure tightly the sealant 4 to the outer glass tube 1, it is desirable that the sealant 4 be made of a material which has substantially the same coefficient of thermal expansion as that of the glass tube 1, for example, an iron-nickel-chromium alloy and be bonded to the outer glass tube 1 by the use of a proper adhesive such as low melting point glass frit. Furthermore, the sealant 4 is attached to the collecting pipe 3 through wax. The outer glass tube 1, combined with the sealant 4, forms a vacuum chamber. A vacuum cavity 5 is defined by the outer glass tube 1 and the sealant 4 and brought into vacuum state with the aid of a vacuum pump connected to a chip tube (not shown). A spacer 6 of ceramic material, for example, is provided to secure the cylindrical fin structure 2 in place in the interior of the outer glass tube 1. Since the heat collecting pipe 3 is off the center of the outer glass tube 1, force resulting from thermal expansion does not act evenly on the sealant 4 and eventually destructs part of the bonding section even though any measure is taken to absorb such uneven force. It is therefore necessary that the glass 1, the sealant 4 and the adhesive 7 have substantially the same coefficient of thermal expansion to keep tight and ever-lasting adhesion. A proper sealant material may be Fe-Ni alloy and 426 alloy in conjuction with soda glass and Kover in conjuction with class II hard glass. A proper adhesive may be low melting point glass having a coefficient of thermal expansion equal to that of the glass tube and the sealant. Howver, in the event that a metallic material such as 426 alloy and Kover is exposed to the atmosphere, it will become rusted and corroded. For these reasons such material is improper for use in solar heat collector assemblies. Anti-rust treatment seems necessary for surfaces of these metallic materials and is actually achieved by plating or deposition of a rust proofing coating. Such treatment is however less reliable for a prolonged period of time.
While the outer glass tube 1 is sealed with the metallic sealant 4, a considerable amount of heat is liberated from the sealant 4 and the heat-collecting pipe 3 and the sealant 4 of a platelike configuration is easily deformable under the influence of the atmospheric pressure.